For realizing an ultrahigh speed optical transmission system of more than 40 Gbit/s, investigations for a polarization multiplexing technology have been actively made. The polarization multiplexing technology is a format of multiplexing two orthogonal polarized signals of same wavelengths, to transmit two independent signals. In the polarization multiplexing technology, since two polarization states can be utilized, it is possible to reduce a baud rate of transmission signal and to increase frequency utilization efficiency.
For an optical transmission system using this polarization multiplexing technology, there have been proposed various types of control technologies relating to polarization tracking mainly in a reception section (refer to Japanese Laid-Open Patent Application Publication No. 2002-344426). It has been known that, in the polarization multiplexing optical transmission system, degradation of transmission characteristics, which is caused by a fiber nonlinear effect and polarization mode dispersion (PMD), differs according to pulse timing between orthogonal polarization components in the polarization multiplexed light (refer to D. van den Borne, et. al., “1.6-b/s/Hz Spectrally Efficient Transmission Over 1700 km of SSMF Using 40×85.6-Gb/s POLMUX-RZ-DQPSK”, Journal of Lightwave Technology, Vol. 25, No. 1, pp. 222-232, Jan. 2007, 2).
To be specific, for pulse (bit) allocation between the orthogonal polarization components in the polarization multiplexed light as illustrated in a left side of FIG. 30, in view of fiber nonlinear proof strength, bit-interleaved polarization multiplexing which shifts pulses of the orthogonal polarization components to each other by a half bit, achieves excellent transmission characteristics (refer to the right side of FIG. 30). On the other hand, in view of PMD proof strength, bit-aligned polarization multiplexing which makes the pulse timing between the orthogonal polarization components in the polarization multiplexed light to be in-phase, achieves the excellent transmission characteristics (refer to the center of FIG. 30). Accordingly, in order to obtain desired transmission characteristics, it is necessary to set the above pulse timing according to a state of the optical transmission system.
However, in the conventional polarization multiplexing optical transmission system, the pulse timing between the orthogonal polarization components is fixed when an apparatus for generating the polarized multiplexed light is initially set. Accordingly, even though a change in system state, such as, time variation in the PMD, wavelength path modification, wavelength spacing modification or the like, is caused, it is impossible to regulate the pulse timing between the orthogonal polarization components according to such a change. Therefore, there is a problem in that the transmission characteristics are considerably degraded depending on the system state.
In order to solve the above problem and to enable the pulse timing between the orthogonal polarization components to be modifiable, there may be considered, for example, a configuration in which a plurality of transmitters corresponding to different pulse timings is previously prepared, and those transmitters are switched to be used according to the system state. However, if the plurality of transmitters is disposed, there are drawbacks of large-scale of the apparatus, complexity thereof and high-cost performance thereof. Further, in a configuration which modifies the pulse timing between the orthogonal polarization components in one transmitter in manual according to the system state, the modifying work of pulse timing takes time, and therefore, in a case where the system state is changed quickly and frequently, it is difficult to cope with such changes in system state.